Asclepias linaria
Asclepias verticillata
hierba del cuervo, needle leaf milkweed, pineneedle milkweed
asclépiade verticillée, eastern whorled milkweed, horsetail milkweed, whorled milkweed
few–numerous, erect, branched, especially distally, 30–70 cm, woody, bark brown to gray, twigs puberulent with curved trichomes, not glaucous, rhizomes absent.
1 (rarely 2 or 3), erect, sparingly branched, arrested vegetative branches absent, 35–90 cm, sparsely puberulent with curved trichomes in lines, not glaucous, rhizomatous.
eventually caducous, alternate, spiral to irregular, sessile, with 1 stipular colleter on each side of leaf base;
blade linear, needlelike, 1.5–4 × 0.1–0.15 cm, chartaceous, base cuneate, margins revolute, apex acute, mucronate, venation obscure, sparsely pilosulous to glabrate, laminar colleters absent.
3–6-whorled, sessile, with 1 stipular colleter on each side of leaf base on a ciliate interpetiolar ridge;
blade linear, 1.5–7 × 0.1–0.2 cm, chartaceous, base cuneate, margins entire, apex acute, mucronate, venation obscure, surfaces glabrous abaxially, puberulent with curved trichomes adaxially, especially on midvein, to glabrate, margins ciliate, laminar colleters absent.
extra-axillary, sessile or pedunculate, 9–30-flowered;
peduncle 0–2.5 cm, puberulent with curved trichomes to pilosulous, with 1 caducous bract at the base of each pedicel.
extra-axillary at upper nodes, pedunculate, 7–28-flowered;
peduncle 0.8–4 cm, puberulent with curved trichomes on 1 side, with 1 caducous bract at the base of each pedicel.
10–14 mm, pilosulous.
6–10 mm, puberulent with curved trichomes.
erect to pendent;
calyx lobes lanceolate to ovate, 2–3 mm, apex acute, sparsely pilosulous to glabrate;
corolla green to cream, often tinged red or purple, lobes reflexed with spreading tips, elliptic, 3.5–5 mm, apex acute, glabrous abaxially, minutely hirtellous at base adaxially, 1 margin ciliate;
gynostegial column 0.2–0.5 mm;
fused anthers brown, obconic, 1–1.5 mm, wings right-triangular, closed, apical appendages ovate, erose;
corona segments cream, sometimes with greenish or purplish dorsal stripe, subsessile or sessile, cupulate, 2.5–3 mm, exceeding style apex, apex obtuse to rounded, glabrous, internal appendage rod-shaped, slightly exserted, glabrous;
style apex shallowly depressed, green.
erect to spreading;
calyx lobes lanceolate, 1.5–2 mm, apex acute, puberulent with curved trichomes to glabrate;
corolla pale green to cream or ochroleucous, sometimes tan-tinged, lobes reflexed with spreading tips, elliptic, 3–4 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially;
gynostegial column 1–1.2 mm;
fused anthers green, columnar, 1.2–1.5 mm, wings narrowly right-triangular, closed, apical appendages deltoid;
corona segments cream, stipitate, cupulate, dorsally flattened, 1.5–2 mm, exceeded by style apex, apex obtuse, margin shallowly lobed (sometimes obscure) to sharply toothed proximally, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous;
style apex shallowly depressed, green to greenish cream.
naviculate, ovate, 5–6 × 3–4 mm, margin very narrowly winged, faces rugulose, the concave one conspicuously so;
coma 1.5–2 cm.
ovate, 6–7 × 4–5 mm, margin winged, faces smooth;
coma 2.5–3.5 cm.
erect on upcurved pedicels, ovoid, 3.5–5 × 0.6–1 cm, apex acuminate, smooth, glabrous.
erect on straight pedicels, narrowly fusiform, 6–11 × 0.4–0.8 cm, apex acuminate to attenuate, smooth, glabrous.
= 22.
= 22.
Asclepias linaria
Asclepias verticillata
Asclepias linaria is arguably the most distinctive milkweed species in the Americas. It is the only species with woody stems and the only one to form hemispherical shrubs with needlelike leaves. Small plants with few stems are easily mistaken for seedling conifers. This species is widespread and occupies a great variety of habitats in Mexico. It enters the flora area in southeastern Arizona and in southwestern New Mexico only in the Peloncillo Mountains (Hidalgo County); its occurrence in the flora area is evidently relictual. In the region, it is restricted to lower reaches of protected canyons that ameliorate aridity and freezing temperatures, sites that harbor other tropical and subtropical species reaching their northern limits. The plants are often quite floriferous and attract an abundance of Hymenoptera, Lepidoptera, and Diptera.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Asclepias verticillata is parapatric with the closely related and morphologically similar A. linearis, A. pumila, and A. subverticillata. It can be difficult to distinguish from these relatives where their ranges overlap. Similarities with A. linearis and A. subverticillata are discussed under those species; no definitive hybrids between A. verticillata and these species have been documented. The characteristic marginal corona segment tooth is often reduced to a shallow lobe in western populations of A. verticillata, which complicates distinguishing this species from A. subverticillata, and which suggests past introgression. A widely disjunct collection of A. verticillata was made in Arizona, well within the range of A. subverticillata, for Plants of the Hopis (Millspaugh 176 [F]); persistence of the species in Arizona has not been documented by additional collections. Hybrids with A. pumila are usually readily detected because the parental species are distinct in leaf arrangement and internode length (whorled and distant nodes in A. verticillata versus alternate and congested in A. pumila). These hybrids often have mixed phyllotaxy and have been documented in Kansas, Montana, South Dakota, and Texas. Asclepias verticillata is strongly rhizomatous and forms dense colonies on roadsides and in prairies; in forests, however, genets are small and solitary stems are common. Like several other milkweed species, it is rare and declining at the northeastern terminus of its range (in Delaware, Massachusetts, New Jersey, Pennsylvania, Rhode Island, and Vermont). It is also considered rare in Saskatchewan, where it has been documented by few specimens. Reports from Wyoming (Crook County) all seem to pertain to A. pumila (B. Heidel, pers. comm.).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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